Gas turbine engines

ABSTRACT

A gas turbine engine blade tip seal consists of a sealing ring which is controlled by means of two annular control members. The members are arranged such that one control member has a relatively rapid thermal response rate and the other control member has a relatively slow thermal response rate, the sealing ring being controlled such that a preferred tip clearance is maintained under varying engine operating conditions.

This invention relates to gas turbine engines and more particularly to asealing arrangement for sealing the blade tips of an "unshrouded" or"shrouded" type of gas turbine engine turbine rotor.

The difficulties of tip sealing unshrouded type turbine rotors has beenwell known for many years. This problem has become worse as the size ofgas turbine engines and their working temperatures have increased. Oneof the main factors which has to be considered when attempting to designa satisfactory sealing arrangement is the matching of the respectivediameters of the turbine rotor and casing at their working temperaturestaking into account the differing coefficients of expansion of thematerials used in the turbine and casing construction.

Consideration must also be given to the fact that when an engine is runup to operating speed the rotor and casing are subjected to severalstages of radial growth. In the first instance the relatively thin rotorblades expand quickly in response to increase in temperature andcentrifugal loading and to this is added the radial growth of the rotordisc due to centrifugal loading. A further stage of radial growth occurswhen the relatively thick rotor disc heats up to operating temperature.During all the aforementioned phases of expansion the casing surroundingthe rotor grows at a steadily decreasing rate during the overall heatingup process. Therefore the tip clearance between the rotor blades and thecasing must be calculated such as to tolerate all relative changes ingrowth of both the entire turbine rotor and the casing.

Furthermore there are other operating conditions for which the tipclearances must be designed to tolerate, for example when the enginespeed is reduced or alternatively the engine is shut down completely. Inthis situation the turbine casing will cool down and contract extremelyquickly whilst the rotor is still relatively hot and still subjected tothe centrifugal effect.

The object of the present invention is to provide a tip seal whichincludes means such that the turbine tip clearance can be controlled ormaintained at an optimum under most engine operating conditions.

According to the present invention a gas turbine engine turbine tipsealing device comprises an annular sealing ring, a first annularcontrol member having means cooperating with the annular sealing ring,said annular control member having a relatively rapid response rate suchthat it expands or contracts quickly in accordance with a temperaturevariation, and a second annular control member having a relatively slowresponse rate such that it expands or contracts relatively slowly inaccordance with a variation in temperature, the arrangement being suchthat upon an increase in temperature occuring on the device the firstannular control member expands relatively rapidly and by virtue of itscooperating means also expands the annular sealing ring, however uponthe first annular control member reaching a particular diameter itcontacts and is restrained from further expansion by the second annularcontrol member such that the sealing ring is then expanded relativelyslowly in accordance with the rate of expansion of the second annularcontrol member, and upon a decrease in temperature occuring upon thedevice the annular sealing ring initially contracts relatively slowly inaccordance with the second annular control member in a first phase ofcontraction, and then relatively quickly in accordance with the firstannular control member in a second phase of contraction.

According to a further aspect of the present invention a gas turbineengine turbine tip sealing device may comprise an annular sealing ring,a first annular control member having means cooperating with the annularsealing ring, said annular control member having a relatively rapidresponse rate such that it expands or contracts quickly in accordancewith a temperature variation, and a second annular control member havingmeans cooperating with the first annular control member, the secondannular control member having a relatively slow response rate such thatit expands or contracts relatively slowly in accordance with a variationin temperature, the arrangement being such that upon an increase intemperature occuring on the device the first annular control memberexpands relatively rapidly and by virtue of its cooperating means alsoexpands the annular sealing ring, however upon the first annular controlmember reaching a particular diameter it contacts and is restrained fromfurther expansion by the second annular control member such that thesealing ring is then expanded relatively slowly in accordance with therate of expansion of the second annular control member, and upon adecrease in temperature occuring upon the device the annular sealingring initially contracts relatively slowly in accordance with the secondannular control member in a first phase of contraction, and thenrelatively quickly in accordance with the first annular control memberin a second phase of contraction until the first annular control memberis restrained from further contraction by the second annular controlmember such that the annular sealing ring will then contract inaccordance with the second annular control member in a third phase ofcontraction.

The annular sealing ring may comprise a plurality of segmented membersadapted to be slidable with respect to each other, or alternatively maybe a continuous ring of resilient material.

The first annular control member may consist of a relatively thinsection cylindrical member having a relatively small mass and the secondannular control member may comprise a relatively thick section cylinderor alternatively may consist of a portion of the engine casing having arelatively large mass.

Preferably the cooperating means provided upon the first annular controlmember comprises an axially extending recess in which a portion of theannular sealing ring is located.

Furthermore the cooperating means provided upon the second annularcontrol member comprises an axially extending spigot which is locatedwith a recess located within the first annular control member.

For better understanding of the invention an embodiment thereof will bemore particularly described by way of example only and with reference tothe accompanying drawings in which:

FIG. 1 shows a diagramatic side view of a ducted fan type gas turbineengine including a broken away casing portion disclosing a diagrammaticembodiment of the present invention.

FIG. 2 shows an enlarged cross-sectional view in greater detail of theembodiment shown diagrammatically at FIG. 1.

Referring to the drawings a gas turbine engine shown generally at 10includes in flow series a fan 12, a compressor section 13, a combustionsection 14, a turbine section 15, the engine terminating in an exhaustnozzle 17. The fan is rotatably mounted within a fan duct 18 which isdisposed radially outwardly and coaxial with the compressor sectioncasing 13b shown generally in the direction of arrow 19 is adiagrammatic embodiment of a turbine tip sealing device made inaccordance with the present invention.

FIG. 2 of the drawings shows an enlarged cross-sectional view of theturbine tip seal device shown generally at arrow 19 in FIG. 1. Thedevice includes a first annular control member 20 which is of relativelythin cross-section such that it has a relatively small mass. The firstannular control member 20 also includes an axially extending spigot 21which is adapted to lie within a groove which is located within theupstream face of a sealing ring 23. The downstream end of the sealingring is located on engine fixed structure 24 by means of a cooperatingspigot and groove arrangement shown generally at 24. The sealing ring 23preferably consists of a plurality of segments which are slidablylocated with respect to each other. Alternatively the sealing ring 23may consist of a resilient material, however both types of sealing ringmay include an abradable lining 25 such as for example honeycomb.

Arranged radially outwardly of the first annular control member 20 islocated a second annular control member 26 which has a relatively thickcross-section and hence a relatively large mass as compared with thefirst annular control member. For convenience the second annular controlmember 26 in this instance takes the form of a separate ring, however incertain circumstances there may be advantages in making it form a partof the engine casing.

A flange portion 27 is secured to the second annular control member 26by means of a plurality of axially extending bolts one of which is shownat 28. The flange portion 27 includes an axially extending spigot 29which is located within a further groove located within the firstannular control member 20 such that during certain modes of the engine'soperation the movement of one annular control member is controlled bythe movement of the other.

When the gas turbine engine is first started from the cold condition theturbine blades will expand quickly due to increase in temperature andalso because of the centrifugal forces acting upon them. Therefore thesealing ring 23 must have the ability to increase in diameter quickly toensure that a clearance is maintained between the turbine blade tips andthe abradable material layer 25. This is achieved by means of the firstannular control member 20 which by virtue of its relatively thincross-section and low mass reacts quickly in accordance with atemperature variation. In this case the temperature increases quicklytherefore the first annular control member 20 will expand and by virtueof the portion 21 cooperating with the seal ring 25 will move the sealring radially outwards.

However after an initial temperature increase and centrifugal forceacting upon the blades in a first rapid growth phase their rate ofradial growth will slow down to a second phase of growth. Therefore toensure that the first control member 20 does not continue to expand tooquickly and so displace the sealing ring to produce an unacceptablylarge seal clearance the internal diameter of the second control member26 is sized such that the clearance shown at 30 between the two membersreduces until the first control member is restrained from further rapidexpansion by the second control member 26.

The turbine rotor and blade however will continue to expand at a slowerrate of expansion or second phase of expansion. In this phase therelatively large mass of the turbine rotor steadily increases intemperature to that of the engine operating temperature. This phase ofthermal growth is therefore matched by the second control member 26which has a relatively larger cross-sectional area and mass than thefirst control member 20 and this exerts a controlling influence upon it.

During deceleration of the engine in the first instance very littlereduction in turbine diameter will occur as, although the temperature ofthe gas stream passing through the turbine will reduce quickly, thiswill only at first effect the turbine blades which will contractrelatively quickly, however they will still be subjected to centrifugalforces due to the continuing rotation of the turbine; thence the rate ofinitial contraction of the turbine will be relatively small.

The temperature of the first annular control member will also be reducedrelatively quickly due to its thin cross-section however it will notimmediately commence reducing in diameter as it is in a state ofcompression due to its engagement with the second control ring 26.

The rate of contraction of the sealing ring 23 will therefore firstly becontrolled by the rate of contraction of the second control member 26during its first phase of contraction.

As the temperature of the turbine rotor and its rotational speedcontinue to fall the speed of contraction of the turbine diameter willincrease to a second phase due to the combined action of the reductionof centrifugal effect and temperature. In this second phase ofcontraction the first control member 20 will have contractedsufficiently to no longer be effected by the second control member 26.The rate of contraction of the sealing ring 23 will therefore becontrolled by the relatively rapid rate of contraction of the firstcontrol member 20.

The turbine will then finally enter a third phase of contraction duringits deceleration, in this phase the contraction is mainly due to therelatively slow cooling large mass of the turbine rotor. To maintain anadequate blade tip clearance therefore the first control member 20 isrestrained from further rapid contraction by means of the spigotprovided upon the member 27 which is rigidly secured to the secondcontrol member 26. Any further contraction of the sealing ring 23 willtherefore be controlled by the second control member which will contractrelatively slowly by virtue of its relatively large mass.

It will be appreciated that by controlling either the relative masses ofthe two control members 20 and 26 or by choosing materials havingdifferent thermal coefficients of expansion or further alternatively bycontrolling the temperature of the environment in which the respectivecontrol members are located their relative rates of expansion andcontraction or the speed at which they respond can be adjusted such asto ensure that the sealing ring may be varied in diameter to maintain anacceptable turbine tip clearance under all engine operating conditions.

Furthermore it will also be understood that although the moreparticularly described embodiment of the present invention includescooperating means comprising the spigot 29 between the two controlmembers 20 and 26, this feature under certain circumstances need not infact be essential to the effective operation of the sealing device. Itis believed that by suitable choice of materials for the control membersor possibly by carefully governing the temperature of the environment inwhich they are located, their respective rates of expansion andcontraction can be matched such as to obviate the necessity forproviding the cooperating means consisting of spigot 29.

We claim:
 1. A gas turbine engine turbine tip sealing device comprisesan annular sealing ring, a first annular control member having meansco-operating with the annular sealing ring, said annular control memberhaving relatively rapid response rate such that it expands or contractsquickly in accordance with a temperature variation, and a second annularcontrol member having a relatively slow response rate such that itexpands or contracts relatively slowly in accordance with variation intemperature the arrangement being such that upon an increase intemperature occurring on the device the first annular control memberexpands relatively rapidly and by virtue of its co-operating means alsoexpands the annular sealing ring and wherein the improvement comprisesin that when the first annular control member reaches a particulardiameter it contracts and is restrained from further expansion by thesecond annular control member such that the sealing ring is thenexpanded relatively slowly in accordance with the rate of expansion ofthe second annular control member, and upon a decrease in temperatureoccurring upon the device the annular sealing ring initially contractsrelatively slowly in accordance with the second annular control memberin a first phase of contraction and then relatively quickly inaccordance with the first annular control member in a second phase ofcontraction.
 2. A gas turbine engine turbine tip sealing device asclaimed in claim 1 which comprises an annular sealing ring, a firstannular control member having means co-operating with the annularsealing ring, said annular control member having a relatively rapidresponse rate such that it expands or contracts quickly in accordancewith a temperature variation, and a second annular control member havingmeans co-operating with the first annular control member, the secondannular control member having a relatively slow response rate such thatit expands or contracts relatively slowly in accordance with a variationin temperature, the arrangement being such that upon an increase intemperature occuring on the device the first annular control memberexpands relatively rapidly and by virtue of its co-operating means alsoexpands the annular sealing ring and wherein the improvement comprisesupon the first annular control member reaching a particular diameter itcontacts and is restrained from further expansion by the second annularcontrol member such that the sealing ring is then expanded relativelyslowly in accordance with the rate of expansion of the second annularcontrol member, and upon a decrease in temperature occuring upon thedevice the annular sealing ring initially contracts relatively slowly inaccordance with the second annular control member in a first phase ofcontraction and then relatively quickly in accordance with the firstannular control member in a second phase of contraction until the firstannular control member is restrained from further contraction by thesecond annular control member such that the annular sealing ring willthen contract in accordance with the second annular control member in athird phase of contraction.
 3. A gas turbine engine turbine tip sealingdevice as claimed in claim 1 which the sealing ring comprises aplurality of segmented members adapted to be slidable with respect toeach other.
 4. A gas turbine engine turbine tip sealing device asclaimed in claim 1 in which the annular sealing ring comprises acontinuous ring of resilient material.
 5. A gas turbine engine turbinetip sealing device as claimed in claim 1 in which the first annularcontrol member consists of a relatively thin section cylindrical memberhaving a relatively small mass and the second annular control member maycomprise a relatively thick section cylindrical member or alternativelymay consist of a portion of the gas turbine engine casing having arelatively large mass.
 6. A gas turbine engine turbine tip sealingdevice as claimed in claim 1 in which the co-operating means providedupon the first annular control member comprises an axially extendingrecess in which a portion of the annular sealing ring is located.
 7. Agas turbine engine turbine tip sealing device as claimed in claim 1 inwhich the co-operating means provided upon the second annular controlmember comprises an axially extending spigot which is located within arecess provided within the first annular control member.